Recently, interest in energy storage technologies has increased. As the energy storage technologies are extended to such devices as cellular phones, camcorders and notebook PCs, and further to electric vehicles, the demand for a high energy density battery used as a power source of such an electronic device is increased. A lithium ion secondary battery is one of the most satisfactory batteries, and numerous studies towards improvements are now in progress actively.
Among the currently used secondary batteries, a lithium secondary battery developed in the early 1990' includes an anode made of carbon material capable of occluding or emitting lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
The lithium secondary battery has an average discharge voltage of about 3.6V to about 3.7V, which exhibits advantageously higher operation voltage than those of other batteries such as alkali batteries or nickel-cadmium batteries. To create such a higher operation voltage, an electrolyte composition should be electrochemically stable in a charging/discharging voltage range from 0 to 4.2V. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed is used as a solvent for the electrolyte. The solute of the electrolyte is usually a lithium salt, such as LiPF6, LiBF4 or LiClO4, which acts as a source for supplying lithium ions in the battery and thus enables the lithium battery to operate.
Lithium ions coming out from the cathode active material such as lithium metal oxide during an initial charging process of a lithium secondary battery move towards the anode active material such as graphite and then are intercalated between the layers of the anode active material. At this time, due to the high reactivity of lithium, the electrolyte reacts with carbon of the anode active material on the surface of the anode active material such as graphite, thereby generating compounds such as Li2CO3, Li2O and LiOH. These compounds form a kind of SEI (Solid Electrolyte Interface) film on the surface of the anode active material such as graphite.
The SET film plays the role of an ion tunnel, which allows only lithium ions to pass. Due to the ion tunnel effects, the SET film prevents organic solvent having high molecular weight from moving together with lithium ions in the electrolyte and being intercalated into layers of the anode active material and thus breaking down the anode structure. Thus, since the electrolyte is not contacted with the anode active material, the electrolyte is not decomposed, and also the amount of lithium ions in the electrolyte is reversibly maintained, thereby ensuring stable charging/discharging.
However, in a thin angled battery, while the above SEI film is formed, gas such as CO, CO2, CH4 and C2H6, generated by decomposition of a carbonate-based solvent, increases the battery thickness during the charging process. In addition, if a battery is left at a high temperature in a fully charged state, the SEI film is slowly broken down due to increased electrochemical energy and thermal energy over time. As a result, side reactions continuously occur between the exposed surface of the anode and surrounding electrolyte. Due to continuous gas generation at this time, an inner pressure of the battery is increased, thereby increasing thickness of the battery, and this may cause problems in electronics such as cellular phones and notebook computers with regard to a high-temperature performance of the battery. In addition, the lithium secondary battery containing a large amount of ethylene carbonate exhibits a more serious problem in inner pressure increase of the battery since the SEI film is unstable. Further, since the ethylene carbonate has a high freezing point (37 to 39° C.) and it is in a solid state at room temperature, it has low ionic conductivity at a low temperature. Thus, a lithium battery using a non-aqueous solvent containing a large amount of ethylene carbonate exhibits poor low-temperature conductivity.
In order to solve the above problems, it has been suggested to use a method of adding a carbonate-based organic additive to the electrolyte so as to change the phase of the SEI film forming reaction. However, it is so far known in the art that, when the above specific compound is added to an electrolyte to improve the battery performance, some areas of performance are improved, but other areas of performance may deteriorate in many cases.
For example, Japanese Laid-open Patent Publication No. H07-153486 discloses a lithium secondary battery using an electrolyte made by adding 0.5 to 50 volume % of γ-butyrolactone to a 1:1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate. However, if γ-butyrolactone is added in this manner, the life cycle of the battery may be shortened though high-rate discharging characteristics at a low temperature are improved.
In addition, Japanese Patent No. 3,032,338 discloses a non-aqueous electrolyte secondary battery containing a ternary system organic solvent composed of an ethylene carbonate, dimethyl carbonate and methyl propionate. However, a linear carbonate such as dimethyl carbonate deteriorates charging/discharging cycle efficiencies of a lithium secondary battery, and methyl propionate deteriorates discharging characteristics since it has a relatively high reactivity with the anode.
In addition, Japanese Laid-open Patent Publication No. 1999-31527 discloses a non-aqueous electrolyte secondary battery containing a ternary system organic solvent composed of a cyclic carbonate, linear carbonate and ethyl propionate. However, this lithium secondary battery shows deteriorated charging/discharging cycle efficiencies due to the linear carbonate. In addition, ethyl propionate has low reactivity with the anode at a normal temperature to exhibit a good discharging characteristic. However, it exhibits a deteriorated discharging characteristic at a high temperature since it reacts with the anode. Accordingly, in the published document, the content of added ethyl propionate is controlled to be 5 volume % or less in order to minimize a self-discharging phenomenon at high temperature storage. Nevertheles, good low-temperature discharging characteristics are not ensured when ethyl propionate is added in a small amount as mentioned above.
Meanwhile, Japanese Patent No. 3,029,271 discloses a lithium secondary battery using a mixed organic solvent in which cyclic carbonate such as propylene carbonate and linear ester carbonate compound such as methyl acetate are mixed. However, methyl acetate also has relatively high reactivity with an anode, so a discharging characteristic decreases.
As mentioned above, it is urgent to develop a non-aqueous electrolyte composition capable of providing a lithium secondary battery that exhibits excellent high-rate charging/discharging characteristics, life cycle, low-temperature discharging characteristics and high-temperature discharging characteristics.